Create and register a custom status code and allow full customization of behavior.

Create and register a custom status code and allow full customization of behavior. The value of allowsEntity
changes the parser behavior: If it is set to true, a response with this status code is required to include a
Content-Length header to be parsed correctly when keep-alive is enabled (which is the default in HTTP/1.1).
If allowsEntity is false, an entity is never expected.